Mitosis is a process by which a complete copy of a duplicated genome is segregated by the microtubule spindle apparatus into two daughter cells. Aurora-kinases, key mitotic regulators required for genome stability, have been found to be overexpressed in human tumors. There is therefore an existing need in the therapeutic arts for compounds which inhibit Aurora-kinases, compositions comprising the inhibitors and methods of treating diseases during which Aurora-kinases are unregulated or overexpressed.
The reversible phosphorylation of proteins is one of the primary biochemical mechanisms mediating eukaryotic cell signaling. This reaction is catalyzed by protein kinases that transfer the g-phosphate group of ATP to hydroxyl groups on target proteins. 518 such enzymes exist in the human genome of which ˜90 selectively catalyze the phosphorylation of tyrosine hydroxyl groups Cytosolic tyrosine kinases reside intracellularly whereas receptor tyrosine kinases (RTKs) possess both extracellular and intracellular domains and function as membrane spanning cell surface receptors. As such, RTKs mediate the cellular responses to environmental signals and facilitate a broad range of cellular processes including proliferation, migration and survival.
RTK signaling pathways are normally highly regulated, yet their over-activation has been shown to promote the growth, survival and metastasis of cancer cells. Dysregulated RTK signaling occurs through gene over-expression or mutation and has been correlated with the progression of various human cancers.
The VEGF receptor (VEGFR) family consists of three RTKs, KDR (kinase insert domain-containing receptor; VEGFR2), FLT1 (Fms-like tyrosine kinase; VEGFR1), and FLT4 (VEGFR3). These receptors mediate the biological function of the vascular endothelial growth factors (VEGF-A, -B, -C, -D, -E and placenta growth factor (PlGF)), a family of homodimeric glycoproteins that bind the VEGF receptors with varying affinities.
KDR is the major mediator of the mitogenic, angiogenic and permeability-enhancing effects of VEGF-A, hereafter referred to as VEGF. Many different cell types are able to produce VEGF, yet its biological activity is limited predominately to the vasculature by way of the endothelial cell-selective expression of KDR. Not surprisingly, the VEGF/KDR axis is a primary mediator of angiogenesis, the means by which new blood vessels are formed from preexisting vessels.
FLT1 binds VEGF, VEGF-B and placental growth factor. FLT1 is expressed on the surface of smooth muscle cells, monocytes and hematopoietic stems cells in addition to endothelial cells. Activation of FLT1 signaling results in the mobilization of marrow-derived endothelial progenitor cells that are recruited to tumors where they contribute to new blood vessel formation.
FLT4 mediates the signaling of VEGF-C and VEGF-D, which mediate formation of tumor-associated lymphatic vessels (lymphangiogenesis). Lymphatic vessels are one of the routes by which cancer cells disseminate from solid tumors during metastasis. The PDGF receptor (PDGFR) family consists of five RTK's, PDGFR-a and -b, CSF1R, KIT, and FLT3.
The a and b isoforms of the platelet-derived growth factor (PDGF) receptors occur as homodimers or a/b heterodimers and are found most commonly on the surface of fibroblasts and smooth muscle cells. PDGFR-b contributes to tumor angiogenesis through the proliferation and migration of pericytes, the peri-endothelial cells that associate with and stabilize immature blood vessels. In gliomas, autocrine PDGFR stimulation, brought about by the co-expression of PDGF and PDGF receptors, mediates tumor cell proliferation and survival.
CSF-1R is encoded by the cellular homolog of the retroviral oncogene v-fms and is a major regulator of macrophage development. Macrophages are frequent components of tumor stroma and have been shown to modify the extracellular matrix in a manner beneficial to tumor growth and metastasis.
KIT is expressed by hematopoietic progenitor cells, mast cells, germ cells and by pacemaker cells in the gut (interstitial cells of Cajal). It contributes to tumor progression by two general mechanisms namely autocrine stimulation by its ligand, stem cell factor (SCF), and through mutations that result in ligand-independent kinase activity.
FLT3 is normally expressed on hematopoietic stem cells where its interaction with FLT3 ligand (FL) stimulates stem cell survival, proliferation and differentiation. In addition to being over-expressed in various leukemia cells, FLT3 is frequently mutated in hematological malignancies with approximately one-third of patients with acute myeloid leukemia (AML) harboring activating mutations.
The identification of effective small compounds which specifically inhibit signal transduction and cellular proliferation by modulating the activity of tyrosine kinases to regulate and modulate abnormal or inappropriate cell proliferation, differentiation, or metabolism is therefore desirable. In particular, the identification of methods and compounds that specifically inhibit the function of a tyrosine kinase which is essential for angiogenic processes or the formation of vascular hyperpermeability leading to edema, ascites, effusions, exudates, and macromolecular extravasation and matrix deposition as well as associated disorders would be beneficial.